The present invention generally relates to systems for operating hydraulic circuits. More particularly, this invention relates to a system and method for pump-controlled cushioning of a hydraulic actuator used to control the position of a working implement on a mobile machine.
Compact excavators, wheel loaders and skid-steer loaders are examples of multi-function machines whose operations involve controlling movements of various implements of the machines. FIG. 1 illustrates a compact excavator 100 as having a cab 101 mounted on top of an undercarriage 102 via a swing bearing (not shown) or other suitable device. The undercarriage 102 includes tracks 103 and associated drive components, such as drive sprockets, rollers, idlers, etc. The excavator 100 is further equipped with a blade 104 and an articulating mechanical arm 105 comprising a boom 106, a stick 107, and an attachment 108 represented as a bucket, though it should be understood that a variety of different attachments could be mounted to the arm 105. The functions of the excavator 100 include the motions of the boom 106, stick 107 and bucket 108, the offset of the arm 105 during excavation operations with the bucket 108, the motion of the blade 104 during grading operations, the swing motion for rotating the cab 101, and the left and right travel motions of the tracks 103 during movement of the excavator 100. In the case of a compact excavator 100 of the type represented in FIG. 1, the blade 104, boom 106, stick 107, bucket 108 and offset functions are typically powered with linear actuators 109-114 (represented as hydraulic cylinders in FIG. 1), while the travel and swing functions are typically powered with rotary hydraulic motors (not shown in FIG. 1).
On conventional excavators, the control of these functions is accomplished by means of directional control valves. However, throttling flow through control valves is known to waste energy. In some current machines, the rotary functions (rotary hydraulic drive motors for the tracks 103 and rotary hydraulic swing motor for the cabin 101) are realized using displacement control (DC) systems, which notably exhibit lower power losses and allow energy recovery. In contrast, the position and velocity of the linear actuators 109-114 for the blade 104, boom 106, stick 107, bucket 108, and offset functions typically remain controlled with directional control valves. It is also possible to control linear hydraulic actuators directly with hydraulic pumps. Several pump-controlled configurations are known, using both constant and variable displacement pumps. Displacement control of linear actuators with single rod cylinders has been described in U.S. Pat. No. 5,329,767 or German Patents DE000010303360A1, EP000001588057A1 and WO002004067969, and offers the possibility of large reductions in energy requirements for hydraulic actuation systems. Other aspects of using displacement control systems can be better appreciated from further reference to Zimmerman et al., “The Effect of System Pressure Level on the Energy Consumption of Displacement Controlled Actuator Systems,” Proc. of the 5th FPNI PhD Symposium, Cracow, Poland, 77-92 (2008), and Williamson et al., “Efficiency Study of an Excavator Hydraulic System Based on Displacement-Controlled Actuators,” Bath ASME Symposium on Fluid Power and Motion Control (FPMC2008), 291-307 (2008), whose contents are incorporated herein by reference.
Hydraulic actuators have a limited position range, or stroke. When the piston of the actuator reaches either end of its stroke, the piston assembly makes contact with the cylinder body and stops. Without some form of cushioning, the impact between the piston and cylinder can cause undesirable wear, vibration and operator discomfort. For some machines, other safety problems such as vehicle instability may result from a sudden actuator stop. To prevent these problems, hydraulic actuators are commonly equipped with viscous dampers called “cushions” that slow the actuator piston near the end of its stroke by forcing the hydraulic fluid through small orifices. If an actuator is not equipped with a cushion, the operator must manually control the actuator velocity to avoid an end-of-stroke impact. However, manually regulating the actuator velocity requires skill and attention.